Energy absorbing PU foam (sometimes called viscoelastic foam) is a type of flexible polyurethane foam which is characterized by a slow recovery from deformation and a high vibration damping. Such properties permit a widespread use for the foam type in medical, packaging, automotive and sporting goods products.
High resilience, water-blown energy absorbing PU foams are described inter alia, in U.S. Pat. Nos. 4,190,712, 4,116,893, 4,282,330 and 4,212,954. In these proposals a variety of polyol mixtures are suggested which often require specific cross-linking agents to maintain a useful, predominantly open celled foam (open cells give greater flexibility to a foam). The proposed foams also have densities in excess of 80 kg/m.sup.3.
In commercial use, the viscoelastic foam sold under the trade name TEMPUR is suggested in British Patent Specification No. 2,290,256 A to find use in mattresses and cushions. The preferred density range for viscoelastic foams for such uses is stated in GB-A-2,290,256 to be 50 to 120 kg/m.sup.3. In the Proceedings of the 35th Annual Polyurethane Technical/Marketing Conference, 1994, pages 661 to 664 in an article entitled "Viscoelastic Slabstock Foam Fundamentals, Properties and Applications", a new foam with trade name SOFTCEL from ARCO Chemical Company is reviewed which is prepared from a specialty polymer polyol, toluene diisocyanate and conventional foam modifier and slabstock intermediates with an additional reinforcing polymer polyol if variation in hardness is required. The prepared foam densities range from 2 pcf to above 4 pcf (around 32 to 64 kg/m.sup.3 or higher).
Low density energy absorbing foams, foams having less than 50 kg/m.sup.3, are commercially attractive, retaining good hardness and hysteresis in a low resilience energy absorbing foam while requiring less raw material than higher density foams.